1. Field of the Invention
The invention relates to a process for the continuous production of sheet metal strips, particularly of steel, as well as to a device to implement this process.
2. Description of the Prior Art
EP 0 311 602 B1 discloses a process for producing thin metal strands, e.g., of steel, with a thickness under 20 mm. In this process, a metallically pure steel strip (mother strip), whose surface is at ambient temperature, is passed vertically through a metal bath from bottom to top or top to bottom. The metal bath can consist of the same material as the mother strip or of material that differs from the mother strip. The retention time of the mother strip in the metal bath is determined as a function of bath temperature so that metal crystals are formed and melt is deposited on the surface of the mother strip, without the mother strip itself melting or already deposited material remelting. In this way, it is possible to produce a strip-type semifinished product with a thickness roughly 6 to 10 times that of the original mother strip. In this process, in contrast to standard continuous casting, solidification occurs not from the outside to the inside, but rather in the opposite direction. For this reason, this form of production of semifinished products is sometimes referred to as inversion casting.
WO 94 29 048 discloses another inversion casting process, in which a thin steel strip is passed through a steel melt from bottom to top and then, upon emerging from the melt, is immediately smoothed on its surface by means of a pair of smoothing rollers. After the smoothing roller pair, the steel strip passes through a cooling zone filled with inert gas, where the strip is cooled in a controlled fashion to improve its material properties.
Because the object of inversion casting is generally to crystallize as much material as possible on the mother strip, the mother strip is usually introduced into the melt at ambient temperature. However, it is not necessarily desirable, particularly when producing metal strips with layers of different materials (composite materials), to attain the greatest possible coating thickness. In the case of composite materials, it is often desirable to produce considerably smaller layer thicknesses, instead of products roughly 3 to 6 times as thick as the mother strip. This can be done by drastically reducing the contact time between the melt and the mother strip. When this is done, however, the bond between the crystallized material and the mother strip is often inadequate, so that complete bonding does not occur with the required reliability. To reduce the growth rate on the surface of the mother strip and, at the same time, still ensure good bonding between the mother strip and the crystallized material, the mother strip can be preheated, so as to lessen its cooling capacity and thus its crystallization potential. This procedure can be used, in particular, to produce multi-layered materials (e.g., carbon steel coated with stainless steel).
It is possible to preheat the mother strip to a particular desired temperature prior to its entry into the melt by arranging a suitable preheating furnace in the form of a continuous furnace in front of the melt container as a separate aggregate. Such furnaces can be heated with fossil energy carriers (e.g., gas or oil) or electrical energy (e.g., an induction furnace). The use of a plasma burner is also conceivable.
Solutions of this type entail relatively high additional equipment expenses, especially since the forward motion speeds for the mother strip are quite high, usually between 10 and 100 m/min. Furthermore, the mother strip introduced into the melt must have a metallically pure surface. This means that preheated mother strips, in particular, must be protected against oxygen, because otherwise re-oxidation will rapidly begin. Oxidized surface areas would jeopardize the required bonding with the crystallized material.